Mobile devices are carried by a user throughout most or all of a day. During the day, the user may encounter many different environments, each with a different background noise characteristic and other acoustic effects. Mobile devices employ noise cancelling to take into account the environmental changes and improve the user's experience while using the mobile device. An adaptive noise cancelling (ANC) algorithm may generate an anti-noise signal to inject into a speaker output to improve the quality of audio produced by the speaker. However, the performance of noise cancelling systems vary with how closely a speaker of the mobile device is placed against the user's ear because the coupling between the user's ear and the speaker varies with distance.
Conventionally, an error microphone is included near the earpiece speaker of a mobile device. The error microphone measures audible signals at the earpiece to monitor the performance of the ANC algorithm. An ANC algorithm may incorporate input from the error microphone as feedback to adjust the ANC algorithm to improve cancellation of environmental noise. However, a single error microphone is only able to measure an acoustic pressure at the user's ear, which may be a misleading indicator of how an acoustic field generated by the speaker is behaving. In particular, the acoustic pressure measured by the error microphone is the acoustic pressure at the earpiece of the mobile device. The acoustic pressure at the earpiece is not necessarily the acoustic pressure at the user's ear drum, where the acoustic field is translated into perceptible noise for the user.